This is a phase I, multicenter, open label trial of the investigational agent cordycepin used in combination with 2'-deoxycoformycin (Pentostatin). There has been no proven use of this drug in humans. The purpose of this research study is to determine the safest dose of these two drugs and to record any effects the combination may have on leukemia. In order to be eligible for this study one must have leukemia and have failed standard therapy. Preliminary laboratory data shows that cordycepin is the active agent in the combination drugs and has been effective in killing leukemia cells and may be beneficial to both adult and children with leukemias since almost all forms of this disease express TdT.